It is not uncommon for military officers to carry a pistol with an empty chamber or even an empty gun. I could have been he drew his pistol and chambered the round because of something he saw or heard when he went into the room, to defend himself?

This could also explain the poorly placed kill shot, the mark getting a pistol into action could have been a serious problem for the whole set up, so the killer took a hurried shot because things did not go as planned.